The present invention relates to a screw compressor, and more particularly to a screw compressor for delivering or feeding gas such as air in a compressed condition.
As shown in FIG. 1, in a typical conventional screw compressor, gas sucked from a suction opening portion 1 is confined in a groove-like space defined by a pair of rotors 2, 2' and a casing 3 and when the rotation of the rotors 2, 2' is progressively advanced, the groove-like space is reduced to a volume which corresponds to a built-in volume ratio of the screw compressor. The gas is then compressed to a ratio corresponding to the built-in volume ratio and the compressed gas is then discharged from a discharge opening portion 4 of the casing.
In the conventional screw compressor, however, when the pressure at the discharge opening is greater than the pressure of the compressed gas within the rotor grooves (under-compression), a rapid backflow of the gas into the grooves results, while when the pressure at the discharge opening is smaller than the pressure of the compressed gas within the rotor grooves (overcompression), a strong discharge of gas into the discharge opening is caused. These rapid changes in flow rate during the discharging operation cause a wide pressure variation at the discharge opening portion. The casing of the compressor is thus directly vibrated owing to the variation in the pressure of the discharged gas, so that noise is generated by the casing. The variation in the pressure also causes vibration in the rotors of the compressor, and the vibrating force from the rotors is then transmitted to the casing via bearings. Further, noise is also generated by the gear and bearing portions due to the vibration of the rotors.
On the other hand, the noise on the discharge side of the screw compressor is directly transmitted to the suction side thereof through a solid member, i.e. the casing. Further, the vibration on the discharge side is propagated to the suction side of the casing through the gas leaking through the gaps defined between the two rotors and between the rotors and the casing. The present inventors recognize through their investigation that the noise at the suction opening portion 1 of the casing is mainly caused by the latter reasons.
Tolerances of the gaps between the two rotors and between the rotors and the casing are determined in consideration of production accuracy (allowances in machining and assembling processes), heat deformation, torsional deformation of the rotors due to axial torque and the like. The smaller the tolerance values of the gaps is, the lower the transmission of the pressure variation from the discharge side to the suction side through the leakage of the gas. However, there are actually restrictions in accuracy due to the reasons stated above so that tolerances cannot be achieved at less than a critical value.
In general, in this type of compressor, the pressure ratio is essentially high and the operating range is normally limited to a high range. The uppermost efficiency point is designed as a target operating point. If a compressor is operated within a range outside of the designed operating point, the reduction in efficiency of the compressor and an increase in the above-described noise caused by vibration are remarkable. On the contrary, a known compressor employs a mechanism referred to as a "slide vane" which acts to increase the operating range. The structure is, however, complicated, and accordingly, this type of compressor is not an essential measure for solving the problem previously described.
Also, as a method for attenuating the pressure variation in a screw compressor, it has been proposed to provide a narrow or fixed size flow passage communicating between a space of a groove defined by the rotor and the casing and a discharge opening or suction opening of the compressor. In this type of compressor, however, the attenuation of the pressure variation could effectively be made within a targeted operating range without substantially decreasing the efficiency of the compressor.